1. Field of Invention
The invention relates generally to photobioreactors and processes to operate and use photobioreactors for the treatment of gases, such as flue gases, and for the production of biomass.
2. Discussion of the Related Art
The power generation industry is coming under increasing pressure to produce electricity from renewable energy sources. Many biofuels meet renewable energy source standards, however, sources of conventional biofuels, such as biomass, biodiesel, and bioethanol, are not uniformly geographically distributed across the nation, and in general, these sources are not located close to power generation facilities.
At the same time, reductions in carbon dioxide emissions and other gas emissions from various sources are becoming increasingly necessary and/or desirable. Typically, capturing carbon dioxide from the flue gas of anthropogenic sources such as electric power plants is expensive.
Photosynthesis is the carbon recycling mechanism of the biosphere. In this process organisms performing photosynthesis, such as plants, synthesize carbohydrates and other cellular materials by CO2 fixation. One of the most efficient converters of CO2 and solar energy to biomass are microalgae, often referred to herein simply as “algae,” which are the fastest growing photoautotrophic organisms on earth and one of nature's simplest microorganisms. In fact, over 90% of CO2 fed to algae can be absorbed, mostly through the production of cell mass. (Sheehan, John; Dunahay, Terri; Benemann, John R.; Roessler, Paul, “A Look Back at the U.S. Department of Energy's Aquatic Species Program: Biodiesel from Algae,” 1998, NERL/TP-580-24190; hereinafter “Sheehan et al. 1998”). In addition, algae are capable of growing in saline waters that are unsuitable for agriculture.
Using algal biotechnology, CO2 bio-regeneration can be advantageous due to the production of useful, high-value products from waste CO2. Production of algal biomass as a method of reducing CO2 levels in combustion gas is an attractive concept because dry algae has a heating value roughly equivalent to coal. Algal biomass can also be turned into a high quality liquid fuel which is similar to crude oil or diesel fuel (“biodiesel”) through thermochemical conversion by known technologies. Algal biomass also can be used for gasification to produce highly flammable organic fuel gases suitable for use in gas-burning power plants. (e.g., see Reed, T. B. and Gaur, S. “A Survey of Biomass Gasification” NREL, 2001; hereinafter “Reed and Gaur 2001”).
Algal cultures also can be used for biological NOX removal from combustion gases. (Nagase Hiroyasu, Ken-Ichi Yoshihara, Kaoru Eguchi, Yoshiko Yokota, Rie Matsui, Kazumasa Hirata and Kazuhisa Miyamoto, “Characteristics of Biological NOX Removal from Flue Gas in a Dunaliella tertiolecta Culture System,” Journal of Fermentation and Bioengineering, 83, 1997; hereinafter “Hiroyasu et al. 1997”). Some algae species can remove NOX at a wide range of NOX concentrations and combustion gas flow rates. Nitrous oxide (NO), a major NOX component, is dissolved in the aqueous phase, after which it is oxidized to NO2 and assimilated by the algal cell. For example, NOX removal using the algae Dunaliella can occur under both light and dark conditions, with an efficiency of NOX removal of over 96% (under light conditions).
Over an 18-year period, the U.S. Department of Energy (DOE) funded an extensive series of studies to develop renewable transportation fuels from algae (Sheehan et al. 1998). In Japan, government organizations (MITI), in conjunction with private companies, have invested over $250 million into algal biotechnology. Each program took a different approach, but because of various problems addressed by certain embodiments of the present invention, none has been commercially successful to date.
A major obstacle for feasible algal bio-regeneration and pollution abatement has been an efficient, yet cost-effective, growth system. DOE's research focused on growing algae in massive open ponds as big as 4 km2. The ponds require low capital input; however, algae grown in open and uncontrolled environments result in low algal productivity. The open pond technology made growing and harvesting the algae prohibitively expensive, because massive amounts of dilute algal waters required very large agitators, pumps and centrifuges. Furthermore, with low algal productivity and large flatland requirements, this approach could, in the best-case scenario, be applicable to only 1% of U.S. power plants. (Sheehan et al. 1998). On the other hand, the MITI approach, with stricter land constraints, focused on very expensive closed algal photobioreactors utilizing fiber optics for light transmission. In these controlled environments, much higher algal productivity was achieved, but the algal growth rates were not high enough to offset the capital costs of the systems utilized. Other examples of closed photobioreactors known in the art include U.S. Pat. Nos. 2,732,663; 4,473,970; 4,233,958; 4,868,123; and 6,827,036.
Burlew (Burlew, John S. “Algal Culture: From Laboratory to Pilot Plant.” Carnegie Institution of Washington Publication 600. Washington, D.C., 1961 (hereinafter “Burlew 1961”)) provides an overview of several designs for algae bioreactors. The bioreactors discussed in Burlew 1961 include the use of glass tubes, open tanks, open trenches, and covered trenches. In these systems, carbon dioxide is fed into a liquid via gas sparging. More recently, Pulz and Scheibenbogen (Pulz O. and Scheibenbogen K. “Photobioreactors: Design and Performance with Respect to Light Energy Input,” Advances in Biochemical Engineering/Biotechnology, 59:pp 124-151 (1998); hereinafter “Pulz 1998”) reviewed algae photobioreactors, and Richmond (Richmond A. ed. “Handbook of Microalgal Culture—Biotechnology and Applied Phycology, Blackwell Publishing, Oxford, UK (2004); hereinafter “Richmond 2004”) reviewed the general state of the art of microalgae culturing, including reactor design. Both references (Richmond 2004 and Pulz 1998) note that open systems, such as natural lakes, circular ponds, and raceway reactors are the predominate commercial technology. Open air systems used for cultivation of algae are also shown in, for example, U.S. Pat. Nos. 3,650,068; 3,468,057; and 4,217,728.